This invention relates generally to the field of communications network and, more particularly, to a system and method for multiplying communications capacity on a multi-channel network by allowing user communication paths on available non-controlled channels without additional hardware by using slave channeling.
Time Domain Multiple Access (TDMA) is the technique of dividing time on a channel into a sequence of frames, each consisting of a number of slots, and allocating one or more slots per frame for a logical communications path. Demand Assigned Multiple Access (DAMA) is a channel access and resource allocation technique that provides the dynamic sharing of one or more channels among multiple users or user networks.
In a DAMA system, user access is provided on an as-needed basis. When users require access, they transmit a request for channel resources on a common control signaling, i.e. orderwire, channel. If channel resources are available, the user is assigned access and can begin communicating. If resources are not available, the user may receive a busy indication or resources could be preempted from a lower priority user. When users complete communication, their channel resources are returned to a pool for reallocation to other users.
TDMA communications systems allow multiple users to communicate through a DAMA multiplexer over a network using a common communications path. A DAMA multiplexer is a TDMA connection between a user baseband device, such as a teletype, telephone, or fax and a communication system, such as a local area network (LAN) or satellite relay system. A DAMA multiplexer retains, sends, and receives data in a holding buffer until the specified time slot for communications occurs. The data is then burst in or out of the buffer at a much higher rate to facilitate a perceived constant communications path.
The DAMA multiplexed data stream is divided into periodic time frames, and each frame is further subdivided into segments as shown in FIG. 1. Some of the segments, e.g. CCOW, RCCOW, support the DAMA protocol. The A, B, and C segments support data communications, and they are further divided into time slots.
The Channel Control Order Wire (CCOW) slot is used to transmit system timing and control information from the channel controller to subscriber units only. The Return Channel Control Order Wire (RCCOW) slot provides limited orderwire capability for DAMA-related subscriber-to-channel controller communications. The Ranging Time slot is the time slot during which the user's DAMA multiplexer determines the range between the user terminal and the satellite to set the transmitter synchronization required for timing. The Link Test slot is used to evaluate the performance of the satellite link.
Data Time slots are the time slots during which users transmit or receive data. There are three segments of time slots in each frame, designated A, B, and C. The number of circuits in each segment depends on the baseband data rate, the forward correction rate, and the transmission burst rate, as influenced by the radio frequency interference (RFI) environment.
In U.S. Pat. No. 6,301,262, Peterson developed algorithms to create DAMA Semi-Automatic Controller (DAMA SAC), an automatic communications channel controller for UHF satellite communications (SATCOM). One channel controller is required for each satellite channel. Each DAMA SAC computer has the capability of controlling up to eight DAMA channels for its given coverage area. Hence, the invention in U.S. Pat. No. 6,301,262 allowed efficient automatic control (AC) mode use of UHF channels and considerably increased UHF SATCOM channel efficiency.
However, even with increased channel efficiency, there remains a need for increased communications capability. The number of controlled channels available for communications is still limited by the number of fielded channel controllers. This system and method for multiplying communications capacity on a TDMA network, using slave channeling, takes advantage of available non-controlled channels, channels that cannot be controlled due to lack of ground control equipment. If a time slot in which to send or receive a message is not available on a control channel and a time slot is available on a slave channel, then users receive and transmit their orderwire on the control channel and send or receive messages on the slave channel. Thus, a slave channel multiplies the communications capability of a network without the need for additional control channels.